We seek to answer two questions: how do neurons become connected during development, and why do they become disconnected during neurodegenerative disease? We previously developed a potential Drosophila model of mammalian neurodegenerative disease by inactivating the protein kinase Cdk5. This is the fly homolog of one of the two main proteins responsible for phosphorylating tau into the form found in the neurofibrillary tangles that are characteristic of many forms of human neurodegeneration. This year, we published data documenting many phenotypic similarities between the neurodegenerative syndrome in this endogenous process of flies and that observed in humans and mice, thus validating our Drosophila mutant as a model of the mammalian disease process. We also extended our analysis of a novel pathological process that occurs early in disease progression in our fly model that had not previously been described in studies of the mammalian diseases. Last year, we found that lack of Cdk5 activity prevents a neuron from segregating the portion of the cell where action potentials initiate (the axon initial segment). This is apt to be relevant to the mechanism of neurodegeneration, since we find that the defective initial segment domain becomes prone to development of gross axonal swellings as the fly ages, and later is the focus of histological abnormalities such as large, vacuolated holes in the tissue. We now find that this is not just a developmental function of Cdk5. This year, we showed that Cdk5 is required chronically for maintenance of the initial segment, as introduction of pharmacologic inhibitors of Cdk5 cause acute loss of the initial segment even after it has formed. We also initiated systematic, genome-wide investigations into the consequences of Cdk5 inactivation. We have used microarrays to perform gene expression profiling of adult flies having, or lacking, Cdk5 activity. We have focused our efforts on early events in degeneration by isolating RNA from young flies, at a time before the onset of overt cellular or organismal defects in the mutants. Preliminary analysis reveals two classes of gene expression changes in mutants lacking Cdk5 activity. We observe alterations in genes directly important for neural function, including sensory transduction, synaptic transmission and ion homeostasis. We also observe changes in the expression of genes associated with a variety of organismal physiological processes we had previously hypothesized to be associated with Cdk5 function, based on our phenotypic analysis. These include proteostasis, redox state, mitochondrial structure and function and stress-sensitivity. The microarray analysis, therefore, serves as a powerful independent validation of the phenotypic analysis. Moreover, as this unbiased, genome-wide array analysis failed to identify major classes of genes not predicted by the earlier studies, these data suggest that we have identified the major functional pathways downstream of Cdk5 that are responsible for its pathological consequences.